It is known that one can determine the titer of a virus by determining the extent of cytopathic effect (CPE) using serial dilutions of the virus to infect a susceptible tissue culture. This technique cannot be used, however, for hepatitis A virus (HAV) which infects cells without producing a visible CPE. Marcus and Carver have described this interference phenomenon for rubella virus, J. Virol., April 1967, pp. 334-343. When cells were infected with rubella virus, the rubella infection rendered the cells refractory to subsequent Bunyamwera virus infection. A difficulty with an interference assay, however, is that not all cell sheets infected with a first virus prevent growth of the second virus with the result that there is no difference in CPE between cells infected with the first virus and uninfected cells. Moreover, an interference assay is not absolutely specific for the virus in question and it is a difficult assay to develop and Perform. Enhancement assays for hog cholera virus have been described by Kumagai et al., J. Immunol. 87:245 (1961) and Matumoto et al.. J. Immunol 87:257 (1961).